Water-insoluble acrylic acid superabsorbent polymers have been in use in absorbent articles, such as diapers, for many years. Such superabsorbent polymers are capable of absorbing many times their weight of water and body fluids and can retain such absorbed liquids under moderate pressure, as measured by absorption under load (AUL). These superabsorbent acrylic polymers have been manufactured by a number of polymerization techniques, including aqueous solution polymerization or bulk polymerization, which maintains an aqueous reaction mixture (or monomer solution) as a single phase until solid particles of polymer are formed. Other techniques for manufacturing superabsorbent acrylic polymer absorbents include, for example, multi-phase polymerization processing techniques, such as inverse emulsion polymerization or inverse suspension polymerization. In these multi-phase techniques, the aqueous acrylic monomer solution is suspended in the form of tiny droplets in a matrix of a water-immiscible, inert organic solvent, such as cyclohexane.
In any of the methods for manufacturing water-insoluble polyacrylic superabsorbent polymers, the polyacrylic acid polymer preferably is neutralized at least about 25 mole percent, more preferably at least about 50 mole percent, and usually about 70-75 mole percent for optimum absorbency. Neutralization may be carried out by neutralizing the acrylic acid monomer before polymerization of the monomer, or the polymer may be neutralized after the polymerization reaction is substantially complete. Examples of post polymerization neutralization are disclosed in U.S. Pat. Nos. 4,062,817 and 4,654,039. After polymerization and internal cross-linking of the monomer, followed by partial neutralization, e.g., 50-100 mole percent neutralization, preferably 70-75 mole percent neutralization, the polymer then is subdivided, e.g., shredded, for more efficient drying, then dried and milled to a desired particle size. The polymer preferably then is surface cross-linked and again dried to form the final product.
In accordance with the manufacturing method of the present invention, it has been found that in using any acrylic acid polymerization technique, as briefly described above, for manufacturing acrylic acid-based, water-insoluble superabsorbent polymers, a substantial number of processing improvements can be realized by post-polymerization neutralization using a solid (e.g., powdered or granular) neutralizing agent that is intimately and homogeneously mixed into the polymer gel after substantial completion of the acrylic acid monomer polymerization reaction. Substantial completion of the acrylic acid monomer polymerization reaction is defined herein to be achieved when the rate of increase in temperature of the polymerizing acrylic acid monomer solution, during the polymerization reaction, decreases to about 5.degree. C. per minute or less, preferably to 2.degree. C. per minute or less, and more preferably to about 1.degree. C. per minute or less.